1. Field of the Invention
The present invention relates to a condition responsive circuit, and particularly to circuitry comprising an electrical thermometer for monitoring the interior temperature of food being cooked.
2. Description of the Prior Art
This invention was conceived as an improvement over the food-temperature monitoring apparatus disclosed in U.S. Pat. No. 3,778,798-Heit which is an improvement of the apparatus disclosed in the application of Stanley B. Welch, Ser. No. 124,325 filed Mar. 15, 1971, now U.S. Pat. No. 3,815,113, both assigned to the General Electric Company, the assignee of the present invention. These food-temperature monitoring devices were developed for use in monitoring the temperature of meat and other foods while being cooked in a baking or roasting oven. These devices provide a needle-like meat probe enclosing a small sensing thermistor, adapted to be driven into the food, in circuit with a programmable unijunction transistor employed as a detector of a Wheatstone bridge having a low voltage AC supply, and a signal alarm buzzer to be actuated upon the reaching of a desired temperature within the meat.
The present invention improves upon the previous inventions by locating the buzzer alarm within the circuit in series with a combination NPN transistor-diode, current switching device such that the current to operate the buzzer does not flow through the PUT or the voltage dividing resistors of the Wheatstone bridge. A resistor in series with the transistor base-collector junction and the buzzer alarm allows a slight back current to flow through the buzzer resulting in a greatly increased amount of magnetomotive force variation therein, allowing a much less critical buzzer air gap. The NPN transistor is turned on by the PUT to allow load current for the buzzer to flow through the collector-emitter circuit during the positive excursion of the AC cycle.
It is therefore a general object of the present invention to provide a circuit, including a programmable unijunction transistor, that is responsive to a condition change such that the change is sensed and translated as a variance in electrical resistance in the circuitry whereby indication means is triggered to signal the condition change.
It is a more particular object of the present invention to provide a low-cost, solid-state electric thermometer with a circuit ensuring the smooth reliable operation of a buzzer alarm upon the reaching of a predetermined, preset temperature within food being cooked, wherein current to the buzzer alarm is limited only by the impedance of its coil and wherein the circuit allows a small back current to flow through the buzzer coil such that the buzzer may be characterized by a fairly noncritical air gap.